System for Growing Crystal Sheets

ABSTRACT

A die for growing a single crystal by an Edge-defined Film-fed Growth (EFG) technique includes a first outer die plate; a second outer die plate; and at least one central die plate positioned between the first outer die plate and the second outer die plate such that at least two capillaries are formed between the first outer die plate and the second outer die plate. First ends of the first outer die plate and the second outer die plate have a slope extending away from at least one of the at least two capillaries to form a growth interface at a top of the die. Second ends of the first outer die plate and the second outer die plate are immersed in a raw material melt provided in a crucible. The raw material melt is configured to travel to the growth interface by capillary flow of the raw material melt through the at least two capillaries.

BACKGROUND OF THE DISCLOSURE Field

The present disclosure generally relates to systems and methods forgrowing crystals from a melt and, more particularly, to systems andmethods for growing crystals having increased thickness.

Description of Related Art

Single crystal sapphire is the material of choice for opticalapplications involving high temperature, shock loading, and abrasion dueto its optical properties, high strength, and abrasion and thermal shockresistance. Various methods have been developed for growing singlecrystal sapphire including the heat exchanger method, Edge-definedFilm-fed Growth (EFG) techniques, and Czochralski (Cz) techniques.

Typical furnaces use with the EFG technique and involve the use of acapillary channel set inside of a die for the supply of liquid to thecrystal growth area. Liquid is supplied to the top of the die from acrucible where a large repository of liquid melt is kept. The kineticsof the supply involve the capillary flow of liquid from the reservoirthrough the thin gap (<1 mm, i.e., a capillary channel) between twoplates. A smaller gap equates to a greater capillary force to overcomethe pull of gravity, but also limits the cross-section through which theliquid can move. If the process tries to grow at higher materialthroughput rates than those allowed by the capillary, then the crystalshrinks from the edges of the die and becomes thinner through thecross-section as well. Accordingly, a need exists for a crystal growthsystem that is configured to supply more liquid melt to the die surfacein order to grow thicker crystal sheets.

SUMMARY OF THE DISCLOSURE

According to an aspect of the present disclosure, provided is a die forgrowing a single crystal by an Edge-defined Film-fed Growth (EFG)technique. The die comprises: a first outer die plate having a generallyrectangular shape; a second outer die plate having a generallyrectangular shape; and at least one central die plate positioned betweenthe first outer die plate and the second outer die plate such that atleast two capillaries are formed between the first outer die plate andthe second outer die plate. First ends of the first outer die plate andthe second outer die plate have a slope extending away from at least oneof the at least two capillaries to form a growth interface at a top ofthe die. Second ends of the first outer die plate and the second outerdie plate are immersed in a raw material melt provided in a crucible.The raw material melt is configured to travel to the growth interface bycapillary flow of the raw material melt through the at least twocapillaries.

According to another aspect of the present disclosure, provided is anapparatus for growing a crystal. The apparatus comprises: a crucibleconfigured to contain a liquid melt; and a die located above the growthcrucible. The die comprises a first outer die plate having a generallyrectangular shape; a second outer die plate having a generallyrectangular shape; and at least one central die plate positioned betweenthe first outer die plate and the second outer die plate such that atleast two capillaries are formed between the first outer die plate andthe second outer die plate. First ends of the first outer die plate andthe second outer die plate have a slope extending away from at least oneof the at least two capillaries to form a growth interface at a top ofthe die. Second ends of the first outer die plate and the second outerdie plate are immersed in a raw material melt provided in a crucible.The raw material melt is configured to travel to the growth interface bycapillary flow of the raw material melt through the at least twocapillaries.

These and other features and characteristics of the apparatus of thepresent disclosure, as well as the methods of operation and functions ofthe related elements of structures and the combination of parts andeconomies of manufacture, will become more apparent upon considerationof the following description and the appended claims with reference tothe accompanying drawings, all of which form a part of thisspecification, wherein like reference numerals designate correspondingparts in the various figures. It is to be expressly understood, however,that the drawings are for the purpose of illustration and descriptiononly and are not intended as a definition of the limits of the device ofthe present disclosure. As used in the specification and the claims, thesingular form of “a”, “an”, and “the” include plural referents unlessthe context clearly dictates otherwise.

BRIEF DESCRIPTION OF THE DRAWINGS

FIGS. 1A and 1B are schematic cross-sectional views of a system forgrowing crystal sheets;

FIG. 2A is a schematic cross-sectional view of a system for growingcrystal sheets illustrating a conventional die with a crystal beinggrown;

FIG. 2B is a portion of the schematic cross-sectional view of FIG. 2Aenlarged for magnification purposes;

FIG. 3A is a schematic cross-sectional view of a die in accordance withone example of the present disclosure;

FIG. 3B is a schematic cross-sectional view of a die in accordance withanother example of the present disclosure;

FIG. 4A is a schematic cross-sectional view of a die in accordance withstill another example of the present disclosure;

FIG. 4B is a schematic cross-sectional view of a die in accordance withanother example of the present disclosure; and

FIG. 5 is a perspective cross-sectional view of a system for growingcrystal sheets in accordance with the present disclosure.

DESCRIPTION OF THE DISCLOSURE

For purposes of the description hereinafter, the terms “upper”, “lower”,“right”, “left”, “vertical”, “horizontal”, “top”, “bottom”, “lateral”,“longitudinal”, and derivatives thereof shall relate to the invention asit is oriented in the drawing figures. However, it is to be understoodthat the invention may assume alternative variations and step sequences,except where expressly specified to the contrary. It is also to beunderstood that the specific devices and processes illustrated in theattached drawings, and described in the following specification, aresimply exemplary embodiments of the invention. Hence, specificdimensions and other physical characteristics related to the embodimentsdisclosed herein are not to be considered as limiting.

Unless otherwise indicated, all ranges or ratios disclosed herein are tobe understood to encompass any and all subranges or subratios subsumedtherein. For example, a stated range or ratio of “1 to 10” should beconsidered to include any and all subranges between (and inclusive of)the minimum value of 1 and the maximum value of 10; that is, allsubranges or subratios beginning with a minimum value of 1 or more andending with a maximum value of 10 or less, such as but not limited to, 1to 6.1, 3.5 to 7.8, and 5.5 to 10.

With reference to FIGS. 1A and 1B, a system growing a crystal, generallydenoted as reference numeral 1, comprises a chamber 3 comprised of aninsulation housing surrounding a crucible 5 configured to contain aliquid melt 7 of material to be crystalized, such as Al₂O₃ powder. A die9 is positioned within the crucible 5. In FIG. 1A the die 9 is extendinginto the page and in FIG. 1B the die is extending across the page. WhileFIGS. 1A and 1B illustrate that one die 9 may be positioned within thecrucible 5, this is not to be construed as limiting the presentdisclosure as any suitable number of dies 9 may be utilized. Die 9, asshown in FIGS. 1A and 1B, includes a pair of die plates 11 having a dieopening 13 and one or more capillaries 15 extending from within thecrucible 5 toward the die opening 13. A seed crystal moving mechanism 17is also provided in the chamber 3 to move the seed crystal(s) 19 awayfrom the die 9 as the crystal 21 is forming.

In general, the die material is selected such that it is chemicallyinert with respect to the liquid melt, but is wetted by the liquid melt.Wetting is indicated by the formation of a meniscus of liquid melt atthe top of die. The size (i.e., height) and shape of the meniscus isdetermined by the surface tension between the molten material and thesolid comprising the die, as well as the density of the liquid melt andgravity. The growth interface, where the crystal is formed, is at thetop surface of the die. Liquid melt is delivered to the top surface ofthe die by capillary flow of liquid rising through the capillaries 15 inthe die 9. With reference to FIGS. 2A and 2B, a typical die 9 includestwo adjacent rectangular die plates 11 extending in and out of the planeof the drawing. The die plates 11 are typically separated by spacers(not shown) so that the die plates 11 are only separated by a smalldistance, typically from 0.05 cm to 0.3 cm, which corresponds to thewidth of the capillary 15. The die opening 13 is defined as the distancefrom the outer edge of one die plate 11 to the outer edge of the other,and determines the thickness of the crystal. The arrangements of the dieplates 11 result in capillaries 15 through which the melt flows from thecrucible 5 to the die opening 13 as denoted by arrows A₁. The melt thenflows to the die opening 13 to form a thin wetting layer of meltedmaterial having boundaries at the outer edges of the die plates 11 asdenoted by arrows A₂. The crystal growth occurs at the solid/liquidinterface at the upper surface of the thin wetting layer of meltedmaterial, with the resulting crystal 21 having a thickness spanning thedie opening 13 (as defined above). After a spreading process from theinitial seed crystal, the final width of the crystal is determined bythe length of the die (in and out of the plane of the drawing). Theheight is determined by how long and how fast the crystal pullingprocess proceeds.

In order to form a crystal using the EFG technique, a seed crystal 19,cut from a previously grown crystal, is brought into contact with thetop of the die 9. Due to the high thermal gradient at the die, portionsof the seed crystal 19 melt on contact with the die, and the thin liquidlayer eventually merges with the capillary liquid. As the seed is pushedfurther down, eventually the solid bottom edge of the seed becomesconformal with the die surface. Once this intimate wetted contact isestablished and a full meniscus is present, the seed can be pulled up,away from the die 9 at a predetermined speed. As the seed crystal iswithdrawn, the melt material crystallizes onto the seed crystal. Whileinitial seed contact is made at only one point on the die (e.g. thecenter), with proper thermal management the meniscus spreads across thelength of the die 9 (in and out of the plane of the drawing). Thespreading is caused by the progressive lateral freezing of the meniscusat the edges of the crystal (not shown in cross-section). The meniscusspreads until it reaches the edge of the die 9. Various physicalprinciples prevent the meniscus from going beyond the edge of the die.As long as the angle formed by the edges of the die 9 is 90° or less,the meniscus is constrained by the edge of the die 9. As the seedcrystal is raised, the meniscus spreads across the surface of the die 9,and the crystal spreads as well (quickly in the left/right directions,slowly in/out of plane). Temperature adjustments to the crucible 5, tothe cavity into which the seed is pulled, and/or adjustments to the seedlift rate are made as required to promote the spread of the crystalacross the surface of the die 9 and to control its growth rate. When thecrystal reaches the shape defined by the edges of the die 9 in bothdimensions, its final form is set by the edges of the die 9.

At that point, a steady state equilibrium is established, and, as longas liquid continues to be delivered to the surface of the die 9 bycapillary action from the melt 7, a crystal of constant cross-sectiondefined by the top surface of the die 9 is grown. The heaters used tomaintain the crucible 5 and the inside of the chamber 3 at theappropriate temperature are chosen by factors such as the size of thecrucible/crystal, the geometry of the crucible (round, rectangular), andthe power requirements. In some instances, induction heating may beemployed. Other systems utilize resistance heating elements. A hybridapproach may also be utilized in which induction coils are used to heata susceptor (usually graphite in the case of sapphire growth), wherebythe susceptor then acts like a resistance heating element that radiatesheat to the crucible.

In addition, the apparatus may also include an afterheater (not shown).The afterheater sits above the crucible 5 and provides an environment inwhich the crystal's temperature drops gradually from the freezing pointto some intermediate value below the freezing point but significantlyabove room temperature, such as around 1800° C. in the case of sapphirecrystal growth. Then, once growth is completed and the crystal has beenseparated from the die, it can be cooled down to room temperature in acontrolled fashion. The afterheater can be either active or passive. Apassive afterheater receives heat radiated or conducted from the hotzone containing the crucible 5. An active afterheater has its own heatsource. This can be controlled separately from the main heat sourceheating the growth crucible 5 and thus be independent, or it can utilizethe same power supply, in which case its time-temperature profile tracksthat of the growth crucible 5 albeit at a lower value.

Generally, the flow of the liquid melt 7 in a die as shown in FIG. 2Apromotes the aggregation of undesirable elements, such as bubbles andimpurities 23, towards the outer surfaces of the grown crystals due tothe sloped edges of the die plates 11 at the die opening 13. This isdesirable since the outer surface of a grown crystal typically needs tobe ground and polished to produce a finished window. However, as shownin FIG. 2B, the above described “normal” flow of the liquid melt may bedisrupted in certain instances. For example, one side of the liquidgrowth interface may collapse due to the temperature at the die opening13 being too cold, thereby causing a jog in the panel. In anotherexample, as shown in FIG. 2B, the temperature at the die opening 13 maybe too high, thereby creating an overly large meniscus allowing vortexconditions as denoted by arrow V. When such vortex conditions exist,bubbles and impurities 23 become incorporated into in the center of thepanel. This is undesirable because there is no process to easily removesuch bubbles and impurities from the final product. Although bubbles andimpurities 23 can be found to incorporate anywhere in the thickness of acrystal, the most common depths are as follows in order ofprevalence: 1) at the surface; 2) <30 microns in from a surface; 3) atthe center; and 4) random other positions

With the above background in mind and with reference to FIG. 3A, asystem for producing thick crystal panels comprises a die 50 having: afirst outer die plate 52 having a generally rectangular shape; a secondouter die 54 plate having a generally rectangular shape; and a centraldie plate 56 positioned between the first outer die plate 52 and thesecond outer die plate 54 such that two capillaries 58 a, 58 b areformed between the first outer die plate 52 and the second outer dieplate 54. A first end 60 of the first outer die plate 52 and a first end62 of the second outer die plate 54 each have a slope 64, 66 extendingaway from the two capillaries 58 a, 58 b to form a growth interface 68at a top of the die 50. Second ends 70 of the first outer die plate 52and the second outer die plate 54 are immersed in a raw material melt 7provided in a crucible 5 (see FIG. 5). In one example, each of slopes64, 66 extend away from the two capillaries at an angle between theslopes 64, 66 and a vertical axis of 90 degrees (flat) down to 20degrees. In one example the angle is in the range of 75 degrees to 45degrees in order to promote the movement of bubbles and other impuritiesto the end of the meniscus. The raw material melt 7 is configured totravel to the growth interface 68 by capillary flow of the raw materialmelt 7 through the two capillaries 58 a, 58 b as denoted by arrows A₁.

While the configuration shown in FIG. 3A allows for production ofthicker panels because more raw material melt 7 is provided to thegrowth interface 68, it can lead to vortex conditions similar to thosedescribed with regard to FIG. 2B caused at the point where the twocapillaries 58 a, 58 b meet. Problems arise when the flows create “deadspots” where the normal laminar flow of material is interrupted and aneddy 72 can form. These eddies 72, or recirculation points, allow notonly the stable presence of bubbles 23 next to a central part of thecrystal, but also the potential to merge smaller bubbles into largerones.

With reference to FIG. 3B, in order to overcome the problems of mergingflows from multiple capillaries 58 a, 58 b, the central die plate 56 maybe provided with a first end 74 having a pair of angled surfaces 76 a,76 b meeting at a point to form a knife-edge. The angled surfaces 76 a,76 b may be provided at an angle of 1 to 30 degrees, for example 5 to 15degrees, relative to the longitudinal axis of the central die plate. Theknife-edge, as shown in FIG. 3B, serves to direct and merge thecompeting flows in an orderly fashion, negating the competitive orbuffeting effects leading to the creation of eddies 72 in FIG. 3A. Inother words, the knife edge at the first end 74 of the central die plate56 provides laminar flow at a point where the two capillaries 58 a, 58 bmerge at the growth interface. In addition, the knife edge may have aradius of curvature of less than 50 microns in order to avoid eddies.For example, an angle of the knife edge may approach zero, within thelimits of the ability to fabricate such delicate edge features over along distance (20+ cm).

With reference to FIG. 5, a plurality of spacers 78 are provided betweenthe first outer die plate 52 and the central die plate 56 and betweenthe second outer die plate 54 and the central die plate 56. The spacers78 extend most of the height of the die 50 but terminate prior to thedie opening. The entire stack (first outer die plate 52, central dieplate 56, second outer die plate 54, and spacers 78) is held together bybolts penetrating from the left side of FIGS. 3A and 3B to the right andnutted on both sides to maintain the tight gap tolerances.

With reference to FIG. 4A, another example of a die 100 for growingthick crystal panels. The die of FIG. 4A is configured to create an eventhicker panel as it is provided with three capillaries and can therebydeliver more raw material melt to the growth interface. The die 100comprises: a first outer die plate 102 having a generally rectangularshape; a second outer die plate 104 having a generally rectangularshape; a first central die plate 106 positioned between the first outerdie plate 102 and the second outer die plate 104; and a second centraldie plate 108 positioned between the first outer die plate 102 and thesecond outer die plate 104. The die plates are arranged such that: afirst capillary 110 a is formed between the first outer die plate 102and the first central die plate 106; a second capillary 110 b is formedbetween the first central die plate 106 and the second central die plate108; and a third capillary 110 c is formed between the second centraldie plate 108 and the second outer die plate 104.

A first end 112 of the first outer die plate 102 and a first end 114 ofthe second outer die plate 104 each have a slope 116, 118 extending awayfrom capillaries 110 a, 110 c. In addition, a first end 120 of the firstcentral die plate 106 has a slope 122 that matches the slope 116 of thefirst end 112 of the first outer die plate 102 and a first end 124 ofthe second central die plate 108 has a slope 126 that matches the slope118 of the first end 114 of the second outer die plate 104. Second endsof the first outer die plate 102 and the second outer die plate 104 areimmersed in a raw material melt 7 provided in a crucible 5. The rawmaterial melt 7 is configured to travel to the growth interface bycapillary flow of the raw material melt 7 through the three capillaries110 a, 110 b, 110 c as denoted by arrows A₁.

While the configuration shown in FIG. 4A allows for production ofthicker panels because more raw material melt 7 is provided to thegrowth interface, it can lead to vortex conditions similar to thosedescribed with regard to FIGS. 2B and 3A caused at the points where thecapillaries 110 a, 110 b, 110 c meet. As discussed above, problems arisewhen the flows create “dead spots” where the normal laminar flow ofmaterial is interrupted and an eddy 72 can form. These eddies 72, orrecirculation points, allow not only the stable presence of bubbles 23next to a central part of the crystal, but also the potential to mergesmaller bubbles into larger ones.

With reference to FIG. 4B, in order to overcome the problems of mergingflows from multiple capillaries 110 a, 110 b, 110 c, the first end 120of the first central die plate 106 comprises an angled knife edge 128configured to direct and merge the raw material melt from capillaries110 a and 110 b at the growth interface and the first end 124 of thesecond central die plate 108 comprises an angled knife edge 130configured to direct and merge the raw material melt from capillaries110 b and 110 c at the growth interface. These angled knife edgesprovide laminar flow at a point where the capillaries 110 a, 110 b, 110c merge at the growth interface. In addition, the first ends 112, 114 ofthe first outer die plate 102 and the second outer die plate 104 eachcomprise a chamfered surface 132, 134 prior to the slope 116, 118. Thechamfered surfaces 132, 134 further help with the smooth merging of theliquid flows. Finally, a plurality of spacers (not shown) are providedbetween the first outer die plate 102 and the first central die plate106, between the first central die plate 106 and the second central dieplate 108, and between the second outer die plate 104 and the secondcentral die plate 108, similarly as shown in FIG. 5.

While examples of a die with two capillaries and three capillaries havebeen discussed hereinabove, this is not to be construed as limiting thepresent disclosure as any suitable number of capillaries may be providedin the die to achieve the desired final panel thickness.

While specific embodiments of the device of the present disclosure havebeen described in detail, it will be appreciated by those skilled in theart that various modifications and alternatives to those details couldbe developed in light of the overall teachings of the disclosure.Accordingly, the particular arrangements disclosed are meant to beillustrative only and not limiting as to the scope of the device of thepresent disclosure which is to be given the full breadth of the claimsappended and any and all equivalents thereof.

1. A die for growing a single crystal by an Edge-defined Film-fed Growth(EFG) technique, the die comprising: a first outer die plate having agenerally rectangular shape; a second outer die plate having a generallyrectangular shape; and at least one central die plate positioned betweenthe first outer die plate and the second outer die plate such that atleast two capillaries are formed between the first outer die plate andthe second outer die plate, wherein first ends of the first outer dieplate and the second outer die plate have a slope extending away from atleast one of the at least two capillaries to form a growth interface ata top of the die, second ends of the first outer die plate and thesecond outer die plate are immersed in a raw material melt provided in acrucible, and the raw material melt is configured to travel to thegrowth interface by capillary flow of the raw material melt through theat least two capillaries.
 2. The die of claim 1, wherein the at leastone central die plate has a first end having a pair of angled surfacesmeeting at a point.
 3. The die of claim 2, wherein the pair of angledsurfaces meeting at a point forms a knife edge.
 4. The die of claim 3,wherein the knife edge has a radius of curvature of less than 50microns.
 5. The die of claim 3, wherein the knife edge is configured toprovide laminar flow at a point where the at least two capillaries mergeat the growth interface.
 6. The die of claim 3, wherein the angledsurfaces are provided at an angle of 1 to 30 degrees relative to thelongitudinal axis of the central die plate.
 7. The die of claim 1,wherein a plurality of spacers are provided between the first outer dieplate and the at least one central die plate and between the secondouter die plate and the at least one central die plate.
 8. The die ofclaim 1, wherein the at least one central die plate comprises a firstcentral die plate and a second central die plate.
 9. The die of claim 8,wherein a first end of the first central die plate has a slope thatmatches the slope of the first end of the first outer die plate and afirst end of the second central die plate has a slope that matches theslope of the first end of the second outer die plate.
 10. The die ofclaim 9, wherein the first end of the first central plate comprises anangled knife edge configured to direct and merge the raw material meltfrom at least two of the capillaries at the growth interface and thefirst end of the second central plate comprises an angled knife edgeconfigured to direct and merge the raw material melt from at least twoof the capillaries at the growth interface.
 11. The die of claim 10,wherein the first ends of the first outer die plate and the second outerdie plate each comprise a chamfered surface prior to the slope.
 12. Thedie of claim 8, wherein a plurality of spacers are provided between thefirst outer die plate and the first central die plate, between the firstcentral die plate and the second central die plate, and between thesecond outer die plate and the second central die plate.
 13. Anapparatus for growing a crystal, the apparatus comprising: a crucibleconfigured to contain a liquid melt; and a die located above the growthcrucible, the die comprising: a first outer die plate having a generallyrectangular shape; a second outer die plate having a generallyrectangular shape; and at least one central die plate positioned betweenthe first outer die plate and the second outer die plate such that atleast two capillaries are formed between the first outer die plate andthe second outer die plate, wherein first ends of the first outer dieplate and the second outer die plate have a slope extending away from atleast one of the at least two capillaries to form a growth interface ata top of the die, second ends of the first outer die plate and thesecond outer die plate are immersed in the liquid melt provided in thecrucible, and the liquid melt is configured to travel to the growthinterface by capillary flow of the raw material melt through the atleast two capillaries.
 14. The apparatus of claim 13, wherein the atleast one central die plate has a first end having a pair of angledsurfaces meeting at a point.
 15. The apparatus of claim 14, wherein thepair of angled surfaces meeting at a point forms a knife edge.
 16. Theapparatus of claim 15, wherein the knife edge has a radius of curvatureof less than 50 microns.
 17. The apparatus of claim 15, wherein theknife edge is configured to provide laminar flow at a point where the atleast two capillaries merge at the growth interface.
 18. The apparatusof claim 15, wherein the angled surfaces are provided at an angle of 1to 30 degrees relative to the longitudinal axis of the central dieplate.
 19. The apparatus of claim 13, wherein a plurality of spacers areprovided between the first outer die plate and the at least one centraldie plate and between the second outer die plate and the at least onecentral die plate.
 20. The apparatus of claim 13, wherein the at leastone central die plate comprises a first central die plate and a secondcentral die plate.
 21. The apparatus of claim 20, wherein a first end ofthe first central die plate has a slope that matches the slope of thefirst end of the first outer die plate and a first end of the secondcentral die plate has a slope that matches the slope of the first end ofthe second outer die plate.
 22. The apparatus of claim 21, wherein thefirst end of the first central plate comprises an angled knife edgeconfigured to direct and merge the raw material melt from at least twoof the capillaries at the growth interface and the first end of thesecond central plate comprises an angled knife edge configured to directand merge the raw material melt from at least two of the capillaries atthe growth interface.
 23. The apparatus of claim 22, wherein the firstends of the first outer die plate and the second outer die plate eachcomprise a chamfered surface prior to the slope.
 24. The apparatus ofclaim 20, wherein a plurality of spacers are provided between the firstouter die plate and the first central die plate, between the firstcentral die plate and the second central die plate, and between thesecond outer die plate and the second central die plate.